Automotive air conditioning compressors of the swash plate type reciprocate a plurality of pistons back and forth within close fitting cylinder bores, compressing refrigerant vapor between the ends of the pistons and a valve plate. A swash plate drive mechanism inherently creates a side load on the reciprocating pistons which, if not adequately resisted, can lead to piston cocking within the cylinder bore, and the vibration, noise and scuffing that results therefrom. Larger capacity compressors often have two headed or two ended pistons, which are driven by a centrally located swash plate. An example may be seen in commonly assigned U.S. Pat. No. 4,351,227. As one end of each piston is driven forward, its opposite end is pulled back, and vice versa. Such designs have the advantage of excellent piston stability, because the two piston ends are widely spaced apart and provide mutual support. This allows the front end of the pistons to be cylindrical, solid and relatively axially short.
Smaller volume compressors have one sided pistons, the sliding ends of which are located all on one side of the swash plate. This provides much less inherent piston stability, and requires extra piston axial length to get an equivalent degree of sliding support surface area. One known means for increasing the effective axial length and sliding surface area of the piston without a large increase in mass is to simply make the piston longer, but hollow. An example may be seen in U.S. Pat. No. 4,145,163. As disclosed there, the type of mechanical interconnection between the swash plate and the piston is a rod with a ball and socket at each end. This necessitates that the hollow piston be open at the back end, in order to clear the horizontally extending rod, meaning that the piston is not as stiff as it would be if it were closed at each end. The same patent shows another attempt to save piston weight, though it is not explicitly described. The metal thickness necessary in the piston head for the ball and socket is localized in a boss at the center of the piston, so a clearance hole in the valve plate is provided to clear the piston boss. When a different type of interconnection between a one sided piston and the swash plate is used, the so called ball and shoe connection as is shown in U.S. Pat. No. 4,351,227 then there is no rod to clear the back of the piston. The piston may then be made hollow, but closed at each end, and therefore stiffer. Such a piston design must be made in at least two pieces, however, since metal cannot be effectively blow molded. Furthermore, it would be difficult at best to provide suction ports through a hollow piston closed at both ends.